It's been turned into an endless well of reaction gifs, so I guess if "lolwut" deserves to be here than "lgtm" deserves to be here.
OTOH, lgtm is pretty much just a GitHub thing.
Draft entry (since this is my first attempt at writing a meme entry, I expect to be given guidance on it)
About
LGTM ("Looks Good To Me") is a positive response to a request for approval, particularly code reviews for software. It's associated with GitHub, though it actually predates it, and it's the most popular comment text on the website.
Origin
I've found uses of the thing in 2008 According to the other blog post, it's been used on mailing lists since long before then.
The rubber stamp
The earliest memetic (as opposed to just being a cliche) use of LGTM is the Rubber Stamp, designed to contrast the official-ness of the stamp with the unofficial sound of the phrase "looks good to me." It also reflects the idea that the reviewer looked at the code but hasn't attempted to test or prove it correct.
LGTM rubber stamps have been made since at least 2012.
Reaction gifs
lgtm.in became a dedicated host for LGTM reaction images. It was created in July 2013, according to the Wayback Machine
- Google Trends